The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device characterized by a process for forming a thin insulating film mainly composed of barium strontium titanate.
Since electronic devices have been manufactured precisely and integrated considerably highly, the function of the electronic device cannot easily be realized by simply devising the circuit structure. The electronic devices include, for example, semiconductor memories, such as an SRAM (Static Random Access read write Memory) having a circuit structure formed by combining a plurality of transistors to perform an operation for storing information. The semiconductor memories include an EEPROM (Electrically Erasable and Programmable Read Only Memory) and a DRAM (Dynamic Random Access Memory) which has a circuit structure constituted by combining transistors and capacitors to perform an operation for storing information. The foregoing semiconductor memories cannot easily be realized by combining conventional MOS transistors and MOS capacitors because the area of the memory cell constituted by the above-mentioned devices has been reduced.
In particular, a semiconductor memory of a type comprising MOS capacitors must prevent reduction in the S/N ratio of a read signal even if the minimum design rule of the device is reduced. However, a predetermined capacity of the capacitor cannot easily be maintained.
In addition to realizing the function of the electronic device by simply devising the circuit structure, use of a thin film made of a functional material, that is, use of the characteristics of the material becomes advantageous.
As a thin insulating film for a MOS capacitor, employment of a thin insulating film has been considered which is made of a functional material, for example, Ba.sub.x Sr.sub.1-x TiO.sub.3 (BST) or PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT) (where 0&lt;x&lt;1) having a dielectric constant higher than that of a silicon oxide film and that of a silicon nitride film/silicon oxide laminate film (an NO film). Moreover, a device, such as an FRAM (Ferroelectric Random Access read write Memory), which operates on a novel principle, has been suggested.
BST is a promising material for making a DRAM capacitor dielectric film which encounters a difficulty in maintaining a sufficiently large capacitor area. The difficulty is created if the degree of integration is raised in order to realize a dielectric constant, the value of which is not smaller than hundreds at ambient temperatures.
When a capacitor device of a semiconductor integrated circuit having a high degree of integration is manufactured by using a ferroelectric film, a chemical-vapor deposition method is a suitable method as a technique for forming the ferroelectric film.
That is, employment of the CVD (chemical vapor deposition) method enables precise control of the composition and reproducibility of the process, that is, an excellent step coverage to be obtained. Therefore, the reliability and the like of the electronic device can significantly be improved.
To form a thin BST film, which is a multi-element thin metal-oxide film, the forming process is usually performed under a mass-transfer rate limited condition. Since the CVD method under the mass-transfer rate limited condition encounters deterioration in the step coverage, employment of a CVD method under a kinetically limited condition for forming the thin BST film has been suggested (Jpn. Pat. Appln. KOKAI Publication No. 7-50104).
The CVD film forming process under the kinetically limited condition which enables a satisfactory step coverage is performed at a deposition temperature of 500.degree. C. or lower. Since the melting point of the BST is 1000.degree. C. or higher, a thin BST film having satisfactory crystallinity cannot easily be formed. Therefore, the thin BST film formed by the CVD method has usually been annealed at the crystallizing temperature or higher (Japanese Patent Laid-Open No. 7-58292 and Japanese Patent Laid-Open No. 9-219497).
However, the method, which has the steps of forming a film by the CVD method under the kinetically limited condition and crystallizing the thin BST film, encounters a problem in that the dielectric constant of the thin BST film is reduced excessively as compared with that of a thin BST film formed at the crystallizing temperature (for example, 700.degree. C. or higher) or higher. The reason for this lies in that the conventional deposition process comprising the steps of forming a film by the CVD method and annealing for crystallizing the film has the following problem.
That is, a deposition process of the foregoing type usually suffers from a problem which arises in a process for annealing the thin BST film formed by a CVD apparatus in an independent annealing apparatus. The problem arises in that the dielectric constant of the thin BST film is reduced considerably because the surface of the thin BST film, which is sensitive to influences of an environment, is exposed to outside air when the thin BST film is transferred from the CVD apparatus to the annealing apparatus.
To overcome the above-mentioned problem, a cluster tool, as shown in FIG. 7, comprising a CVD chamber 81 and a annealing chamber (annealing chamber) 82, has usually been employed. Thus, the thin BST film is transferred between the two chambers in a vacuum state or the same is conveyed in a controlled atmosphere. As an alternative to employment of the cluster tool, a method may, of course, be employed in which the thin BST film is transferred between independent apparatuses by a wafer carrier having a controlled atmosphere.
However, all of the above-mentioned conventional methods have a problem which arises in a process for transferring a BST deposited wafer from the CVD chamber (the apparatus) to the annealing chamber (the apparatus). There arises the problem in that the temperature of the substrate (the temperature of the wafer) is temporarily lowered to the deposition temperature or lower.
That is, as shown in FIG. 8, the semiconductor substrate is caused to have a standard heat history when the moving method is employed which uses the cluster tool structured as shown in FIG. 7 and comprising the CVD chamber 81 and the annealing chamber 82. The standard history inevitably encounters lowering of the temperature of the substrate to a level lower than the deposition temperature by the CVD method when the transfer between the chambers is performed.
Therefore, the crystalline structure of the thin BST film formed at a temperature not higher than 500.degree. C., which is under a kinetically limited condition for forming the BST film by the CVD method, is an amorphous state. Even if BST has been crystallized, the crystalline structure is in a metastable state. That is, if the temperature of the structure is lowered below a critical level lower than the deposition temperature, the thin BST film undesirably forms a stable phase at the low temperature. The stable phase inhibits complete crystallization. Therefore, even if the annealing at a high temperature is performed, there arises a problem in that sufficient crystallinity cannot be obtained. If annealing of BST is sequentially performed without lowering the temperature to a level lower than the deposition temperature, excellent crystallinity can be obtained.
If the temperature of the substrate is lowered during transfer between the chambers, there arises a problem in that the adhesion between the thin BST film and a lower ground (for example, a Ru film) deteriorates. Thus, the thin BST film separates undesirably.
As described above, the suggestion has been made that the conventional method of forming a thin BST film is performed such that the thin BST film is formed by the CVD method under a kinetically limited condition in a state in which the wafer is not exposed to outside air after which annealing is performed to crystallize the thin BST film. The above-mentioned method comprising the step of moving the wafer having a CVD film formed by the above-mentioned method from the CVD chamber to the annealing chamber. Therefore, the foregoing method, however, suffers from problems in that satisfactory crystallinity cannot be obtained and that the formed film is peeled because the temperature of the substrate is lowered during the transfer.